Optical and Structural Properties of Composites Based on Poly(urethane) and TiO2 Nanowires

This article’s objective is the synthesis of new composites based on thermoplastic polyurethane (TPU) and TiO2 nanowires (NWs) as free-standing films, highlighting their structural and optical properties. The free-standing TPU–TiO2 NW films were prepared by a wet chemical method accompanied by a thermal treatment at 100 °C for 1 h, followed by air-drying for 2 h. X-ray diffraction (XRD) studies indicated that the starting commercial TiO2 NW sample contains TiO2 tetragonal anatase (A), cubic Ti0.91O (C), and orthorhombic Ti2O3 (OR), as well as monoclinic H2Ti3O7 (M). In the presence of TPU, an increase in the ratio between the intensities of the diffraction peaks at 43.4° and 48° belonging to the C and A phases of titanium dioxide, respectively, is reported. The increase in the intensity of the peak at 43.4° is explained to be a consequence of the interaction of TiO2 NWs with PTU, which occurs when the formation of suboxides takes place. The variation in the ratio of the absorbance of the IR bands peaked at 765–771 cm−1 and 3304–3315 cm−1 from 4.68 to 4.21 and 3.83 for TPU and the TPU–TiO2 NW composites, respectively, with TiO2 NW concentration equal to 2 wt.% and 17 wt.%, indicated a decrease in the higher-order aggregates of TPU with a simultaneous increase in the hydrogen bonds established between the amide groups of TPU and the oxygen atoms of TiO2 NWs. The decrease in the ratio of the intensity of the Raman lines peaked at 658 cm−1 and 635 cm−1, which were assigned to the vibrational modes Eg in TiO2 A and Eg in H2Ti3O7 (ITiO2-A/IH2Ti3O7), respectively, from 3.45 in TiO2 NWs to 0.94–0.96 in the TPU–TiO2 NW composites, which indicates that the adsorption of TPU onto TiO2 NWs involves an exchange reaction of TPU in the presence of TiO2 NWs, followed by the formation of new hydrogen bonds between the -NH- of the amide group and the oxygen atoms of TixO2x-mn, Ti2O3, and Ti0.91O. Photoluminescence (PL) studies highlighted a gradual decrease in the intensity of the TPU emission band, which is situated in the spectral range 380–650 nm, in the presence of TiO2 NW. After increasing the TiO2 NW concentration in the TPU–TiO2 NW composite mass from 0 wt.% to 2 wt.% and 17 wt.%, respectively, a change in the binding angle of the TPU onto the TiO2 NW surface from 12.6° to 32° and 45.9°, respectively, took place.

Here, we used XRD to uncover information about the crystalline planes of TiO 2 NWs. We show that the NWs contain tetragonal TiO 2 anatase (A), cubic Ti 0.91 O (C), orthorhombic Ti 2 O 3 (OR), and monoclinic H 2 Ti 3 O 7 (M). To highlight the potential changes in the chemical structure of TPU and TiO 2 NWs, we show the correlated studies by Raman scattering and FTIR spectroscopy. According to our previous study, these characterization methods are valuable tools to highlight the exchange reaction of TPU in the presence of BaTiO 3 nanoparticles [43]. Using scanning electron microscopy (SEM), we show the fibrous structure of the TPU-TiO 2 NW composites. To assess the binding angle of TPU onto the TiO 2 NW surface, we performed anisotropic PL measurements. Our research allows an understanding of TPU's adsorption process onto the TiO 2 NW surface. Recently, we demonstrated that TPU shows, at an excitation wavelength of 350 nm, a photoluminescence (PL) band with maximum at 410 nm [43]. We also analyzed the influence of TiO 2 NWs on TPU PL properties.

Materials
TPU was purchased from the Elastollan-BASF Chemical Company (Cleveland, OH, USA), while TiO 2 NWs and N,N -dimethyl formamide (DMF, 99.8%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). According to the TiO 2 NWs specification sheet, the diameter and length of the NWs were~10 nm and~10 µm.

Synthesis Method of TPU-TiO 2 NW Composites
The free-standing-film TPU-TiO 2 NW composites were prepared by the wet chemistry method as follows: (a) we dissolved 0.5 g TPU in 16 mL DMF under ultrasonication; (b) in each solution of TPU in DMF (0.5 g/16 mL), we added 50 or 100 mg of TiO 2 NW; (c) the dispersion of TiO 2 particles in the TPU solution was performed under ultrasonication for 20 min; (d) the TiO 2 suspensions in the solutions of TPU in DMF were poured into a petri vessel and subjected to a thermal treatment for 1 h at a temperature of 100 • C for DMF evaporation; and (ed) we dried the TPU samples with different TiO 2 concentrations, i.e., 2 wt.% and 17 wt.%, in air for 2 h until the free-standing films' mass remained constant.
The TPU free-standing films were prepared as above without adding the TiO 2 NWs. Figure 1 shows the schematic synthesis method for TPU-TiO 2 NW composites as free-standing films.

Materials
TPU was purchased from the Elastollan-BASF Chemical Company (Cleveland, OH, USA), while TiO2 NWs and N,N′-dimethyl formamide (DMF, 99.8%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). According to the TiO2 NWs specification sheet, the diameter and length of the NWs were 10 nm and 10 μm.

Synthesis Method of TPU-TiO2 NW Composites
The free-standing-film TPU-TiO2 NW composites were prepared by the wet chemistry method as follows: (a) we dissolved 0.5 g TPU in 16 mL DMF under ultrasonication; (b) in each solution of TPU in DMF (0.5 g/16 mL), we added 50 or 100 mg of TiO2 NW; (c) the dispersion of TiO2 particles in the TPU solution was performed under ultrasonication for 20 min; (d) the TiO2 suspensions in the solutions of TPU in DMF were poured into a petri vessel and subjected to a thermal treatment for 1 h at a temperature of 100 °C for DMF evaporation; and (ed) we dried the TPU samples with different TiO2 concentrations, i.e., 2 wt.% and 17 wt.%, in air for 2 h until the free-standing films' mass remained constant.
The TPU free-standing films were prepared as above without adding the TiO2 NWs. Figure 1 shows the schematic synthesis method for TPU-TiO2 NW composites as free-standing films.

X-ray Diffraction Analysis
The XRD patterns of the TiO2 NWs and TPU-TiO2 NW composites, which have a TiO2 NW concentration equal to 2 wt.% and 17 wt.%, respectively, were carried out using a Bruker D8 Advance diffractometer (Bruker, Hamburg, Germany) in Bragg-Brentano geometry, which was equipped with a Cu tube, and which had a Cu Kα-line of λ = 1.5406 Å.

Fourier-Transform Infrared (FTIR) Spectroscopic Analysis
The IR spectra of TPU and the TPU-TiO2 NW composites, which had TiO2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively, were recorded using an FTIR spectrophotometer Vertex 80 model from Bruker (Billerica, MA, USA).

FT-Raman Spectroscopic Analysis
The Raman spectra of TPU and the TPU-TiO2 NW composites, which have TiO2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively, were recorded with an FT-Raman spectrophotometer MultiRam model from Bruker at an excitation wavelength of 1064 nm (Ettlingen, Germany).

X-ray Diffraction Analysis
The XRD patterns of the TiO 2 NWs and TPU-TiO 2 NW composites, which have a TiO 2 NW concentration equal to 2 wt.% and 17 wt.%, respectively, were carried out using a Bruker D8 Advance diffractometer (Bruker, Hamburg, Germany) in Bragg-Brentano geometry, which was equipped with a Cu tube, and which had a Cu K α -line of λ = 1.5406 Å.

Fourier-Transform Infrared (FTIR) Spectroscopic Analysis
The IR spectra of TPU and the TPU-TiO 2 NW composites, which had TiO 2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively, were recorded using an FTIR spectrophotometer Vertex 80 model from Bruker (Billerica, MA, USA).

FT-Raman Spectroscopic Analysis
The Raman spectra of TPU and the TPU-TiO 2 NW composites, which have TiO 2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively, were recorded with an FT-Raman spectrophotometer MultiRam model from Bruker at an excitation wavelength of 1064 nm (Ettlingen, Germany).

Scanning Electron Microscopy and Energy-Dispersive X-Ray Analysis
Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analysis of TPU and the TPU-TiO 2 NW composites, which have TiO 2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively, were achieved with a Zeiss Gemini 500 field-emission scanning electron microscope and a Zeiss EVO 50 XVP system (Zeiss, Oberkochen, Germany) equipped with a Bruker EDS detector, respectively.

Results and Discussion
3.1. Morphological Properties of TiO 2 NWs and the TPU-TiO 2 Composites Figure 2 shows the SEM images of TiO 2 NWs, as well as TPU and the TPU-TiO 2 NW composites, which have TiO 2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively.

Scanning Electron Microscopy and Energy-Dispersive X-Ray Analysis
Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analysis of TPU and the TPU-TiO2 NW composites, which have TiO2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively, were achieved with a Zeiss Gemini 500 field-emission scanning electron microscope and a Zeiss EVO 50 XVP system (Zeiss, Oberkochen, Germany) equipped with a Bruker EDS detector, respectively. Figure 2 shows the SEM images of TiO2 NWs, as well as TPU and the TPU-TiO2 NW composites, which have TiO2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively. According to Figure 2a, the TiO2 NWs diameter varies between 8 and 11 nm. In contrast with Figure 2b, which shows an SEM image of TPU, in the case of Figure 2c,d, one can observe that the TPU-TiO2 NW composites show a fibrous structure, while the TiO2 NW diameter varies in the case of the TPU-TiO2 NW composites that have TiO2 NW concentrations of 2 wt.% and 17 wt.%, respectively, between 11 and 24 nm and 9 and 18 nm, respectively. The apparent increase in the diameter of TiO2 NWs is caused by the adsorption of polymer on the surface of TiO2 nanoparticles. Figure 3 shows the EDS spectra of According to Figure 2a, the TiO 2 NWs diameter varies between 8 and 11 nm. In contrast with Figure 2b, which shows an SEM image of TPU, in the case of Figure 2c,d, one can observe that the TPU-TiO 2 NW composites show a fibrous structure, while the TiO 2 NW diameter varies in the case of the TPU-TiO 2 NW composites that have TiO 2 NW concentrations of 2 wt.% and 17 wt.%, respectively, between 11 and 24 nm and 9 and 18 nm, respectively. The apparent increase in the diameter of TiO 2 NWs is caused by the adsorption of polymer on the surface of TiO 2 nanoparticles. Figure 3 shows the EDS spectra of TiO 2 NWs, as well as the TPU and the TPU-TiO 2 NW composites, which have a TiO 2 NW concentration equal to 2 wt.% (c) and 17 wt.%, respectively. TiO2 NWs, as well as the TPU and the TPU-TiO2 NW composites, which have a TiO2 NW concentration equal to 2 wt.% (c) and 17 wt.%, respectively. As we expected, Figure 3a,c,d proved the presence of Ti, which in the case of the TPU-TiO2 NW composites confirms the embedding TiO2 NWs in the TPU matrix, inducing a fibrous structure in the free-standing films. Figure 4 shows the XRD patterns of TPU and the TPU-TiO2 NW composites, which have TiO2 NW concentrations equal to 2 wt.% and 17 wt.%, respectively. Figure 4a highlights the XRD patterns of TiO2 NWs peak at 24.2°, 29.7°, 43.4°, 48°, 59.9°, and 66.5° in two theta. According to the standard International Centre for Diffraction Data (ICDD) database, the first two peaks situated at 24.2° and 29.7° belong to the crystalline (110) and (310) planes of the monoclinic (M) H2Ti3O7 [44], PDF no.00-041-0192, while the peaks localized at 43.4° and 48° were assigned to the (200) plane in Ti0.91O, which has a cubic (C) crystalline structure [PDF no.04-004-2981], and the (200) plane in TiO2 of the type tetragonal anatase (A) [45,46], PDF no. 00-021-1272, respectively. The peaks around 60.2° and 66.3° in 2θ belong to the (501) and (404) planes, respectively, of a Ti2O3 phase with an orthorhombic (OR) crystalline structure (PDF04-018-9746). As we expected, Figure 3a,c,d proved the presence of Ti, which in the case of the TPU-TiO 2 NW composites confirms the embedding TiO 2 NWs in the TPU matrix, inducing a fibrous structure in the free-standing films.  Figure 4b). This decrease in the intensity of the peak at 48-48.3 • can only be explained if an interaction of TiO 2 NWs with PTU occurs, when we estimate that the formation of suboxides takes place. To prove this claim, correlated Raman scattering and FTIR spectroscopy studies are presented below.   Figure 4b). This decrease in the intensity of the peak at 48°-48.3° can only be explained if an interaction of TiO2 NWs with PTU occurs, when we estimate that the formation of suboxides takes place. To prove this claim, correlated Raman scattering and FTIR spectroscopy studies are presented below.  (Figure 5a). They are assigned to the vibrational modes of the N-H bond, C-H bond, higherorder aggregates, stretching C(O)-OC, CO stretching in the ether group, C-N bond, C-H bond, urethane group, C-C + C=C bonds in the benzene ring, hydrogen-linked urethane carbonyl group (C=O), the free carbonyl group, the antisymmetric and symmetrical vibrational modes of the CH bonds, and a partial inter-and intramolecular hydrogen linkage of the NH groups of the adjacent urethane segments [47][48][49][50][51][52][53], respectively.  Figure 4b) and 17 wt.% (red curve in Figure 4b).   Figure 4b) and 17 wt.% (red curve in Figure 4b).

Vibrational Properties of TPU and the TPU-TiO 2 NW Composites
Compared with Figure 4a, in the case of the TPU-TiO2 NW composites with TiO2 N concentrations equal to 2 wt.% and 17 wt.%, one observes that: (i) the gradual increase the added TiO2 NWs is clearly visible at the NWs' specific 2θ angles; (ii) the peak in range 15-25° (Figure 4b) belongs to TPU [43]; (iii) a shift in the peaks from 43.4° and (Figure 4a Figure 4b). This decrease in the intensity of the peak at 4 48.3° can only be explained if an interaction of TiO2 NWs with PTU occurs, when we e mate that the formation of suboxides takes place. To prove this claim, correlated Ram scattering and FTIR spectroscopy studies are presented below.  signed to the following vibrational modes: O-C=O in-plane deformation, out-of-plane benzene-ring deformation, out-of-plane C-H wagging, C-C skeletal stretching in alkane group, C-O-C, urethane amide, urethane amide III, deformation of the C-H bond in urethane amide III, symmetric stretching of N=C=O + deformation of CH2 group, stretching of the bonds C-C + C=C in aryl ring, and stretching of CH bond in aromatic structure [54,55], respectively.   [58], B1g in TiO2 A [56], A1g in H2Ti3O7 [44], and Eg in H2Ti3O7, respectively [44]. Increasing the concentration of TiO2 NWs in the mass of the TPU-TiO2 NW composites induces an increase in the intensity of the Raman lines peaking at 278-280 cm −1 , an upshift of the Raman line from 455 cm −1 (Figure 6b) to 457-461 cm −1 (Figure 6a2,a3), and a change in the profile of the Raman line at 658 cm −1 (Figure 6b). The insert in Figure 6b shows that the Raman line at 658 cm −1 displays an asymmetric profile to small wavenumbers as a consequence of the presence of a Raman line peaked at 635 cm −1 , belonging to the vibrational mode Eg of H2Ti3O7 [56]. Regarding   (Figure 6a 1 ), and they are assigned to the following vibrational modes: O-C=O in-plane deformation, out-of-plane benzene-ring deformation, out-of-plane C-H wagging, C-C skeletal stretching in alkane group, C-O-C, urethane amide, urethane amide III, deformation of the C-H bond in urethane amide III, symmetric stretching of N=C=O + deformation of CH 2 group, stretching of the bonds C-C + C=C in aryl ring, and stretching of CH bond in aromatic structure [54,55], respectively. Figure 6b shows that the main Raman lines of  [44], and E g in H 2 Ti 3 O 7 , respectively [44]. Increasing the concentration of TiO 2 NWs in the mass of the TPU-TiO 2 NW composites induces an increase in the intensity of the Raman lines peaking at 278-280 cm −1 , an upshift of the Raman line from 455 cm −1 (Figure 6b) to 457-461 cm −1 (Figure 6a 2 ,a 3 ), and a change in the profile of the Raman line at 658 cm −1 (Figure 6b). The insert in Figure 6b shows that the Raman line at 658 cm −1 displays an asymmetric profile to small wavenumbers as a consequence of the presence of a Raman line peaked at 635 cm −1 , belonging to the vibrational mode E g of H 2 Ti 3 O 7 [56]. Regarding the Raman spectra of TiO 2 NWs, the ratio between the intensity of the Raman lines peaked at 658 cm −1 and 635 cm −1 , which were assigned to the vibrational modes E g in TiO 2 A and E g in H 2 Ti 3 O 7 (I TiO2-A /I H2Ti3O7 ), respectively, is equal to 3.45. Regarding the Raman spectra of the TPU-TiO 2 NW composites with the TiO 2 NW concentrations of 2 wt.% and 17 wt.%, the I TiO2-A /I H2Ti3O7 ratio is equal to 0.94 (Figure 6a 2 ) and 0.96 (Figure 6a 3 ), respectively. The decrease in the value of the I TiO2-A /I H2Ti3O7 ratio in the case of the TPU-TiO 2 NW composites indicates a diminution of TiO 2 A in the TiO 2 NWs mass. This fact can be explained by taking into account the exchange reaction of TPU according to Scheme 1, which is followed by the formation of new hydrogen bonds between the NH bonds of the amide groups and the oxygen atoms of Ti x O 2x-mn (Scheme 2).

Vibrational Properties of TPU and the TPU-TiO2 NW Composites
the Raman spectra of TiO2 NWs, the ratio between the intensity of the Raman lines peaked at 658 cm −1 and 635 cm −1 , which were assigned to the vibrational modes Eg in TiO2 A and Eg in H2Ti3O7 (ITiO2-A/IH2Ti3O7), respectively, is equal to 3.45. Regarding the Raman spectra of the TPU-TiO2 NW composites with the TiO2 NW concentrations of 2 wt.% and 17 wt.%, the ITiO2-A/IH2Ti3O7 ratio is equal to 0.94 (Figure 6a2) and 0.96 (Figure 6a3), respectively. The decrease in the value of the ITiO2-A/IH2Ti3O7 ratio in the case of the TPU-TiO2 NW composites indicates a diminution of TiO2 A in the TiO2 NWs mass. This fact can be explained by taking into account the exchange reaction of TPU according to Scheme 1, which is followed by the formation of new hydrogen bonds between the NH bonds of the amide groups and the oxygen atoms of TixO2x-mn (Scheme 2). The reaction products of Scheme 1 can be described as follows: (a) the first corresponds to a macromolecular compound with amide groups in the repeating units; (b) the second corresponds to a macromolecular compound characterized by the repeating units having acetate groups; and (c) the third corresponds to suboxide TixO2x-mn. Similar to Scheme 2, new hydrogen bonds emerge between the -NH-bond of the TPU amide group and the oxygen atoms of TixO2x-mn. Such hydrogen bonds can be invoked to occur between the -NH-bonds of the TPU amide groups and the oxygen atoms of Ti2O3 and Ti0.91O.
Summarizing these results, the exchange reaction presented in Scheme 1 is confirmed by the change in the ratio between the intensities of the peaks at 43.4°-43.6° and 48°-48.3° from 0.78 (Figure 4a) to close to 1, i.e., 1.03 (blue curve in Figure 4b) and 0.96 (red curve in Figure 4b), which indicates the emergence of suboxide TixO2x-mn, as well as the decrease in the ratio between the intensity of the Raman lines peaked at 658 cm −1 and 635 cm −1 , which is assigned to the vibrational modes Eg in TiO2 A and Eg of H2Ti3O7, from 3.45 to 0.94-0.95 in the case of TPU and the TPU-TiO2 NW composites ( Figure 6). This indicates that there is a decrease in the higher-order aggregates as a consequence of the emergence of new the ITiO2-A/IH2Ti3O7 ratio is equal to 0.94 (Figure 6a2) and 0.96 (Figure 6a3), respectively. The decrease in the value of the ITiO2-A/IH2Ti3O7 ratio in the case of the TPU-TiO2 NW composites indicates a diminution of TiO2 A in the TiO2 NWs mass. This fact can be explained by taking into account the exchange reaction of TPU according to Scheme 1, which is followed by the formation of new hydrogen bonds between the NH bonds of the amide groups and the oxygen atoms of TixO2x-mn (Scheme 2). The reaction products of Scheme 1 can be described as follows: (a) the first corresponds to a macromolecular compound with amide groups in the repeating units; (b) the second corresponds to a macromolecular compound characterized by the repeating units having acetate groups; and (c) the third corresponds to suboxide TixO2x-mn. Similar to Scheme 2, new hydrogen bonds emerge between the -NH-bond of the TPU amide group and the oxygen atoms of TixO2x-mn. Such hydrogen bonds can be invoked to occur between the -NH-bonds of the TPU amide groups and the oxygen atoms of Ti2O3 and Ti0.91O.
Summarizing these results, the exchange reaction presented in Scheme 1 is confirmed by the change in the ratio between the intensities of the peaks at 43.4°-43.6° and 48°-48.3° from 0.78 (Figure 4a) to close to 1, i.e., 1.03 (blue curve in Figure 4b) and 0.96 (red curve in Figure 4b), which indicates the emergence of suboxide TixO2x-mn, as well as the decrease in the ratio between the intensity of the Raman lines peaked at 658 cm −1 and 635 cm −1 , which is assigned to the vibrational modes Eg in TiO2 A and Eg of H2Ti3O7, from 3.45 to 0.94-0.95 in the case of TPU and the TPU-TiO2 NW composites ( Figure 6). This indicates that there is a decrease in the higher-order aggregates as a consequence of the emergence of new The reaction products of Scheme 1 can be described as follows: (a) the first corresponds to a macromolecular compound with amide groups in the repeating units; (b) the second corresponds to a macromolecular compound characterized by the repeating units having acetate groups; and (c) the third corresponds to suboxide Ti x O 2x-mn .
Similar to Scheme 2, new hydrogen bonds emerge between the -NH-bond of the TPU amide group and the oxygen atoms of Ti x O 2x-mn . Such hydrogen bonds can be invoked to occur between the -NH-bonds of the TPU amide groups and the oxygen atoms of Ti 2 O 3 and Ti 0.91 O.
Summarizing these results, the exchange reaction presented in Scheme 1 is confirmed by the change in the ratio between the intensities of the peaks at 43.4-43.6 • and 48-48.3 • from 0.78 (Figure 4a) to close to 1, i.e., 1.03 (blue curve in Figure 4b) and 0.96 (red curve in Figure 4b), which indicates the emergence of suboxide Ti x O 2x-mn , as well as the decrease in the ratio between the intensity of the Raman lines peaked at 658 cm −1 and 635 cm −1 , which is assigned to the vibrational modes E g in TiO 2 A and E g of H 2 Ti 3 O 7 , from 3.45 to 0.94-0.95 in the case of TPU and the TPU-TiO 2 NW composites ( Figure 6). This indicates that there is a decrease in the higher-order aggregates as a consequence of the emergence of new hydrogen bonds between the -NH-, which belongs to the TPU amide groups, and the oxygen atoms of Ti x O 2x-mn , Ti 2 O 3 , and Ti 0.91 O.  (Figure 7c 1 ). This decrease in the PL band indicates that TiO 2 NWs are PTU PL quenching agents. According to G. Strat et al., the redshift of the TPU's PL band results from the luminescence centers belonging the small-order aggregates [59], which in our case appear as a consequence of the reactions shown in Schemes 1 and 2. Figure 7 shows the PL spectra of TPU and the TPU-TiO2 NW composites have a TiO2 NW concentration equal to 2 wt.% and 17 wt.%. The increase in the TiO2 NW concentration in the TPU-TiO2 NW composites mass from 0 wt.% to 17 wt.% induces a shift in the maximum level of the emission band from 416 nm (Figure 7a1) to 457 nm ( Figure 7b1) and 480 nm (Figure 7c1), as well as a decrease in the intensity of the PL band from 1.55 × 10 7 counts/sec (Figure 7a1) to 2.76 × 10 6 counts/sec ( Figure 7b1) and 2.2 × 10 6 counts/sec ( Figure  7c1). This decrease in the PL band indicates that TiO2 NWs are PTU PL quenching agents. According to G. Strat et al., the redshift of the TPU's PL band results from the luminescence centers belonging the small-order aggregates [59], which in our case appear as a consequence of the reactions shown in Schemes 1 and 2.   Figure 7. PL spectra of TPU (a1), PTU-TiO2 NWs 2% (b1), and PTU-TiO2 NWs 17% (c1). Anisotropic PL of TPU (a2), PTU-TiO2 NWs 2% (b2), and PTU-TiO2 NWs 17% (c2). In (a2,b2,c2), blue and red curves correspond to PL spectra recorded when measurement geometry for emission and excitation polarizers are both in horizontal (HH) and vertical (VV) position.

Photoluminescence of TPU and the TPU-TiO2 NWs Composites
Using the mathematic protocol reported in [60], the calculated values of the anisotropy (r) and binding angle (θPL) for TPU are 0.3712 and 12.6°, respectively, while the PTU-TiO2 NW composite has a TiO2 NW concentration equal to 2 wt.%, which is 0.2315 and 32°, and the PTU-TiO2 NW composite has a TiO2 NW concentration of 17 wt.%, which is Using the mathematic protocol reported in [60], the calculated values of the anisotropy (r) and binding angle (θ PL ) for TPU are 0.3712 and 12.6 • , respectively, while the PTU-TiO 2 NW composite has a TiO 2 NW concentration equal to 2 wt.%, which is 0.2315 and 32 • , and the PTU-TiO 2 NW composite has a TiO 2 NW concentration of 17 wt.%, which is 0.0905 and 45.9 • . These values indicate that increasing the TiO 2 NW concentration in the PTU-TiO 2 NW composite mass results in an increase in θ PL of TPU onto the TiO 2 NW surface. As shown above, the θ PL values are different from 0 • , which suggests that the TPU's excitation and emission transition dipoles are not parallel with the TiO 2 NW plane. The orientation of TPU onto the TiO 2 NW surface must consider the hydrogen bonds established between TiO 2 NWs and TPU as well as the products of TPU's exchange reaction with TiO 2 NWs.

Conclusions
In this work, we reported new results concerning the optical and structural properties of TPU-TiO 2 NW composites as free-standing films. Our results highlight the following conclusions: (i) using X-ray diffraction, we demonstrated that TiO 2 NWs contain TiO 2 anatase (A), Ti 0.91 O and Ti 2 O 3 have cubic (C)-and orthorhombic (OR)-type crystalline structures, respectively, while H 2 Ti 3 O 7 has a monoclinic (M)-type structure; (ii) in the case of TPU-TiO 2 NW composites, with TiO 2 NW concentration 2 wt.% and 17 wt.%, the increase in the intensity of the diffraction peak localized at 43.2 • indicated the formation of titanium suboxides; (iii) according to studies using FTIR spectroscopy, the interaction of TPU with TiO 2 NWs involved a decrease in the higher-order aggregates of TPU simultaneous with an increase in the hydrogen bonds established between the TPU amide groups and oxygen atoms of TiO 2 NWs, facts that were highlighted by the variation of the ratio between the absorbance of the IR bands peaking at 765-771 cm −1 and 3304-3315 cm −1 from 4.68 to 3.83 when the concentration of TiO 2 NWs in the composite mass was 0 wt.% and 17 wt.%; (iv) according to Raman spectroscopy, the decrease in the ratio between the intensity of the Raman lines peaked at 658 cm −1 and 635 cm −1 , which were assigned to the vibrational modes E g in TiO 2 A and E g in H 2 Ti 3 O 7 (I TiO2-A /I H2Ti3O7 ), respectively, from 3.45 in TiO 2 NWs to 0.94-0.96 in the TPU-TiO 2 NW composites, indicating that the adsorption of TPU onto TiO 2 NWs involves an exchange reaction of TPU in the presence of TiO 2 NWs, which is followed by the formation of new hydrogen bonds between the -NH-of the amide group and the oxygen atoms of Ti x O 2x-mn , Ti 2 O 3 , and Ti 0.91 O; (v) we demonstrated that the TiO 2 NWs are TPU PL quenching agents, which was evidenced by the decrease in the intensity of the emission band of the TPU-TiO 2 NW composite, which was localized in the spectral range 380-650 nm as increasing TiO 2 NW concentration; and (vi) anisotropic photoluminescence studies indicated a preferential orientation of TPU onto the TiO 2 NW surface, such as when increasing the TiO 2 NW concentration in the PTU-TiO 2 NW composite mass from 0 wt.% to 2 wt.% and 17 wt.%, which induced the increase in the polymer binding angle onto the TiO 2 NW surface (θ PL ) from 12.6 • to 32 • and 45.9 • .